Measurements using Optic and RF Waves

Chapter 5Passive Remote Sensing at SubmillimeterWavelengths and THz¹

 

 

 

5.1. Introduction

During recent years, considerable progress has been made in instrumental techniques and the physics of heterodyne receivers, in particular for submillimeter wave radioastronomy where high detection sensitivity has been reached up until several THz. Today, the observations are rather limited by the quality of the sites because of electromagnetic noise and atmospheric absorptions (Figure 5.1). The advances in receiver technologies justify the choice of sites on high mountains and the use of airplanes, stratospheric balloons or satellites. ALMA (Atacama Large Millimeter Array), the most advanced ground observatory and HERSCHEL, the largest and only space facility dedicated to the submillimeter and far infrared part of the spectrum, include both superconducting heterodyne receivers and will provide an unprecedented combination of sensitivity, angular resolution and spectral resolution at the shortest radio wavelengths during the current decade¹.

Applications of these submillimeter radiometers using heterodyne detection largely exceed the field of radioastronomy and also concern planetology like physicochemistry of cometary gases and planetary atmospheres (Mars, etc.), remote sensing and weather forecasting by satellite (vertical survey of the atmosphere in the presence of cloud cover) and the study of the minor components of the atmosphere (limb survey). There are also civil and military applications, ...